The invention relates to a hydraulic hybrid powertrain for vehicles, particularly to a powertrain having an engine-pump for pressurizing a hydraulic system, an accumulator for energy storage, and pump-motors for driving the wheels.
In a hydraulic hybrid powertrain having a prime mover, such as an engine-pump that produces hydraulic flow at system pressure, and one or more pump-motors driving the wheels, it is desirable to operate at low system pressure to maximize the pump-motor efficiency during most operating conditions. There is, however, a mismatch in operating efficiency of the engine-pump and the efficiency of the pump-motors. The engine-pump has its highest efficiency at high system pressures. The pump-motors have their highest efficiency at lower system pressures.
It is desirable to operate at a high system pressure at times of peak demand, to achieve the required power with a smaller pump-motor. Also, system pressure is directly coupled to the stored energy state from regenerative braking. In a hydraulic hybrid, therefore, it is desired that rapid transitions occur between low system pressure and high system pressure, without incurring significant energy loss. This would allow normal operation at a low system pressure, and quick access to a higher peak torque level on demand. A powertrain operating this way would realize a significant improvement in system cycle fuel economy.
In a hydraulic hybrid powertrain, hydraulic flow at system pressure produced by the prime mover is used to drive one or more hydraulic pump-motors. Energy exceeding the current requirements of the powertrain is stored in a hydro-pneumatic accumulator. The pump-motors can provide regenerative braking. Kinetic energy of the vehicle produced by the pump-motors is recovered by a regenerative braking strategy and is stored in the accumulator. That energy can be supplied as required to the drive system from the accumulator. However, the system pressure necessarily decreases while the accumulator supplies this energy to the system. This drop in accumulator pressure reduces the available drive torque from the system.
It is desirable that the engine and storage accumulator are decoupled so that one pump-motor can use the stored energy to drive a first set of wheels, and another pump-motor can be powered by flow from the engine at pressure up to maximum system pressure to drive another set of wheels. This technique makes more total power available and better uses stored energy.
The magnitude of energy stored in an accumulator is approximately proportional to system pressure, and peak tractive output available from the pump-motors is also directly proportional to system pressure. Changing system pressure in this case requires a significant change in stored energy, and also takes time. This requires a compromise between drivability and use of energy storage to improve fuel economy.